Plastic containers that can reliably contain carbonated beverages generating internal pressures as high as 100 psi or more and that can be inexpensively manufactured in attractive shapes pose a technical problem that has received substantial attention by those working in this art.
The spherical shape, which has the greatest ratio of volume to surface area, provides an optimum uniform distribution of wall stresses generated by internal pressures and thus achieves the maximum reliable and effective strength for a given wall material thickness, and, indeed, internal pressures within non-spherically-shaped containers tend to urge the non-spherically-shaped containers toward a spherical shape. A spherical shape is, however, unacceptable as a commercial beverage container because, among other obvious reasons, a sphere has no stable base and cannot effectively use shelf and storage space of retail and wholesale purveyors and manufacturers.
Workers in the art have sought to develop cylindrical plastic beverage containers that can reliably and attractively contain carbonated beverage products, can be inexpensively manufactured, and have stability when filled and unfilled, and an extensive variety of container designs have been developed by those working in the art to meet these needs.
Such containers have most frequently been manufactured from plastic materials such as polyethylene terephthalate (PET) by, for example, blow molding a parison of PET into a mold formed in the shape of the container. The biaxial expansion of PET by blow molding imparts rigidity and strength to the formed PET material, and blow molded PET can provide economically acceptable wall thicknesses, an attractive container with clarity in relatively intricate designs, sufficient strength to contain pressures up to 100 psi and more, and resistance to gas passage that may deplete contained beverages of their carbonation.
One factor that is, however, frequently over looked in container designs of those working in the art is the propensity of PET to succumb to the deleterious effects of stress cracking and crazing, which is manifest as almost imperceptible streaks in the plastic but ultimately can become complete cracks due to stress and other environmental factors. Relatively unstretched portions of a plastic container that have low degrees of crystallinity due to the lack of biaxial expansion, such as the central bottom portion, are particularly susceptible to crazing and stress-cracking. The relatively unstretched central portion of the container bottom is also frequently provided with a plurality of depending feet that are formed with distention-resistant but stress concentrating areas, and the composite effect on such areas of stress and strain due to the internal pressure of the container and external environmental factors, such as exposure to stress cracking agents, (e.g., caustics, water, oils and generally any plastic solvent or softening agent) can lead to crazing, stress-cracking and container bottom failure.
One commercial cylindrical beverage container that seeks to avoid such problems is formed with a full hemispherical bottom portion and provided with a separate plastic base member fastened over the hemispherical bottom portion to provide a stable base for the container. Such containers are in common use for large multi-liter containers for carbonated beverages, even though the provision of a separate plastic base member imposes increased manufacturing and material costs on the cost of each container. Offsetting somewhat the increased costs imposed by the addition of a separate base piece, is the fact that use of a hemispherical bottom portion can permit a reduction in the bottom wall thickness, tending to maximize the containable pressure for a given wall thickness in the bottom portion and reducing the cost of the plastic material in the container portion.
Those working in the art have also generated commercial containers including "champagne" type bases including concave, or "domed" eversion-resisting central bottom portions merging with the cylindrical container sidewalls at an annular ring which forms a stable base for the container. The central domed portion of a champagne-based plastic container generally creates clearance for the gate area of the container which is intended to resist deformation due to the internal pressure of the container but is sensitive to stress cracking. Unfortunately, containers with champagne bases require a greater wall thickness in the base portion to resist the distending and everting forces of the internal pressure and form stress concentrations at the annular base-forming transition between the concave central bottom portion and cylindrical sidewall that are prone to stress cracking and rupture when the container is dropped. One container design addressing this problem is disclosed in U.S. Pat. No. 4,249,666.
Notwithstanding their champagne bases, it is not uncommon, however, particularly during hot summer months, for the bottoms of such commercial containers to distend and increase the internal volume enough to significantly lower the fluid level, creating an unacceptable product presentation to the consumer, and in some cases to expand beyond their intended bases, creating unstable and unacceptable "rockers".
More recently, the use of hemispherical bottom portions and concave champagne-like bottom portions have been combined by workers in the art in designs in which a plurality of feet are formed in the bottom of a blow molded container. These designs frequently seek eversion-resistant concave central bottom portions formed by a plurality of surrounding feet that are interconnected by a plurality of generally convex hemispheric rib portions. Some such container designs providing footed bottles are in commercial usage, particularly in smaller containers for carbonated beverages such as those containing a liter or less.
Such container designs, however, are still subject, in the absence of relatively thick bottom wall portions, to distention of their concave central portions due to high internal pressures that can create "rockers" and significantly increased interior container volume with lower fluid levels, all of which are unacceptable to purchasers. Efforts to increase the eversion and distention resistance of the concave bottom portions of such footed containers with thinner bottom wall thicknesses have frequently led to bottom portions including small radii of curvature and discontinuous and abrupt transitions between adjoining surfaces that provide stress concentration, crazing and stress cracking sites. Some container designs, for example, those of U.S. Pat. Nos. 4,865,206 and 5,353,954, have addressed the problem of stress concentration, stress cracking and impact resistance. None of these container designs is entirely satisfactory in view of cost, manufacturability and reliability.